A cold cathode FED uses electron emissions to illuminate a cathodoluminescent screen and generate a visual image. An individual field emission cell typically includes one or more emitter sites formed on a baseplate. The baseplate in active matrix FEDs typically contains the active semiconductor devices (e.g. field effect transistors) that control electron emissions from the emitter sites. The emitter sites may be formed directly on a baseplate formed of a material such as silicon or on an interlevel conductive layer (e.g., polysilicon) or interlevel insulating layer (e.g., silicon dioxide, silicon nitride) formed on the baseplate. A gate electrode structure, or grid, is typically associated with the emitter sites. The emitter sites and grids are connected to an electrical source for establishing a voltage differential to cause a Fowler-Nordheim electron emission from the emitter sites. These electrons strike a display screen having a phosphor coating, releasing the photons that illuminate the screen. A single pixel of the display screen is typically illuminated by one or more emitter sites.
In a gated FED, the grid is separated from the base by an insulating layer. This insulating layer provides support for the grid and prevents the breakdown of the voltage differential between the grid and the baseplate. Individual field emission cells are sometimes referred to as vacuum microelectronic triodes. The triode elements include the cathode (field emitter site), the anode (cathodoluminescent element) and the gate (grid). U.S. Pat. No. 5,210,472, granted to Stephen L. Casper and Tyler A. Lowrey, entitled "Flat Panel Display In Which Low-Voltage Row and Column Address Signals Control A Much Higher Pixel Activation Voltage", and incorporated herein by reference, describes a flat panel display that utilizes FEDs.
The quality and sharpness of an illuminated pixel site of the display screen is dependent upon the precise control of the electron emission from the emitter sites that illuminate a particular pixel site. In forming a visual image, such as a number or letter, different groups of emitter sites must be cycled on or off to illuminate the appropriate pixel sites on the display screen. To form a desired image, electron emissions may be initiated in the emitter sites for certain pixel sites while the adjacent pixel sites are held in an off condition. For a sharp image, it is important that those pixel sites required to be isolated remain in an off condition. Thus, shifts in the threshold voltage (V.sub.T) (the voltage necessary to turn on the transistor for the pixel) are undesirable; and there is difficulty in maintaining the V.sub.T at a level such that unwanted activation will not occur.